1. Field of the Invention
This invention relates to,. an apparatus for adjusting the inclination of a rotatable optical member or the like, for example, an apparatus for and a method of adjusting the inclination of an optical element or the like which is suitable for a light beam scanning apparatus which effects reading or recording of images by the use of a light beam.
2. Related Background Art
As an example of the prior art, a plan view of a light beam scanning apparatus is shown in FIG. 7 of the accompanying drawings, and a cross-sectional view thereof is shown in FIG. 8 of the accompanying drawings. In these figures, the reference numeral 1 designates a well-known polygon mirror having the side surface thereof comprising a plurality of mirror surfaces 1a. The polygon mirror 1 is held by a mounting seat 2 which is placed above a base plate 3 in spaced apart relationship therewith. A rotary shaft 4 is journaled to the base plate 3, and the mounting seat 2 is slightly in contact with the rotary shaft 4. A plurality of surface fall adjusting screws 5 are threaded into the base plate 3 so as to contact the bottom surface of the mounting seat 2, and a fastening ring member 7 is coupled to the base plate 3 by means of bolts 6. A resilient ring 8 is interposed between the flange portion of the mounting seat 2 and the overlapping portion of the fastening ring member 7.
Rotation of the rotary shaft 4 causes rotation of the base plate 3 fixed to the rotary shaft 4, and the fastening ring member 7 coupled to the base plate 3 rotates following the rotation of the base plate. The rotation is transmitted to the mounting seat 2 by the friction thereof with respect to the fastening ring member 7 through the medium of the resilient ring 8 and thus, the polygon mirror 1 mounted on the mounting seat 2 is rotated.
Now, in the light beam scanning apparatus of this type, it is sometimes the case that the light beam scans a position deviated relative to a desired scanning position and the intervals between scanning lines which should be equal are disturbed. This leads to the problem that an image to be formed cannot be obtained accurately.
As a cause of this, there may be mentioned the surface fall of the mirror surfaces la of the polygon mirror 1 relative to the rotary shaft 4. This surface fall refers to the irregularity of each mirror surface la relative to a predetermined reference surface when the light beam emitted by a light beam generating device from a predetermined position is caused to scan a photosensitive member or the like by the reflection by the mirror surfaces 1a of the polygon mirror 1. If there is such surface fall, the position of the light beam which scans the photosensitive member differs from one mirror surface 1a to another and therefore, the intervals between scanning lines are disturbed and appear as pitch irregularity on the image.
The following may be mentioned as a primary cause of the surface fall. If the polygon mirror 1 is mounted on a rotative driving device while being inclined relative to the rotary shaft 4, the inclination of the next mirror surface 1a differs from a certain reference surface relative to the position of the scanning line scanned by the certain reference surface of the light beam entering from a predetermined position and therefore, the angle of reflection changes and the position of the scanning line differs. Accordingly, it is necessary to mount the polygon mirror 1 so that it is orthogonal to the rotary shaft 4, in other words, each mirror surface 1a is parallel to the rotary shaft 4. Actually, however, it is most difficult to mount the polygon mirror 1 on the rotary shaft 4 so that the former is orthogonal to the latter and so, it becomes necessary to adjust the mounting, i.e., to adjust the surface fall.
In the prior art, the level of the tip ends of the three adjustment screws 5 is adjusted to thereby adjust the inclination of the mounting seat 2 on which the polygon mirror 1 is mounted.
The reason why three adjustment screws 5 are used is that even if more than three screws are used, it is difficult to apply them to the mounting seat 2 with a uniform force.
However, the direction of inclination of the polygon mirror 1 relative to the rotary shaft 4 ranges over 360.degree. and may assume any direction and therefore, it is very difficult to adjust the inclination by the use of only the three adjustment screws 5. Also, in in the prior art, consideration is given by using the resilient ring 8 so that no strain is produced in the mounting seat 2 and further in the polygon mirror 1. However, adjustment of the bolts 6 is also necessary to prevent a non-uniform load from being applied to such a resilient member.
In addition, the polygon mirror 1 bears chiefly the primary scanning in the light beam scanning and is rotated at a high speed and therefore, the resilient ring 8 is partly deformed or the positional deviation of the resilient ring 8 occurs due to the centrifugal force or the like during the high-speed rotation of the polygon mirror, and the adjusted surface becomes inclined again relative to the rotary shaft 4. Further, the resilient ring 8 has resiliency and therefore it may be expanded or contracted by the influence of a variation with time or heat or the like, and stable adjustment of the surface fall is difficult to maintain.
Particularly, under the requirement for images of high quality, it is said that if the intervals between scanning lines deviate about 0.2%, such deviation is visually recognized. If the intervals between scanning lines are 80-100 .mu.m, the pitch irregularity of 0.2 .mu.m will not be allowed, and the use of a readily deformable resilient member in the light beam scanning apparatus becomes a very unstable element with respect to the surface fall.